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Evaporation of water from the Earths surface forms one part of the water cycle. At 100C, the boiling point, all water will rapidly be turned to vapour, for the energy supplied to the water is enough to break apart all the molecular bonds in water. At temperatures between 100C and 0C, only some of the molecules in the water have enough energy to escape to the atmosphere and the rate at which water is converted to vapour is much slower.

The rate of evaporation will depend upon a number of factors. Rates increase when temperatures are higher. An increase of 10C will approximately double the rate of evaporation. The humidity of the surrounding air will also influence evaporation. Drier air has a greater "thirst" for water vapour than humid, moist air. It follows therefore, that the presence of wind will also increase evaporation. On still days, water evaporating to the air remains close to its source, increasing the local humidity. As the moisture content of the air increases, evaporation will diminish. If, however, a steady flow of air exists to remove the newly formed vapour, the air surrounding the water source will remain dry, "thirsty" for future water.